The present invention relates to a radio communication system correlated with, for example, UWB (Ultra Wide Band) communication or the like using signals of a ultra wide frequency band.
Recently, in accordance with rapid increase of information content, there are increasing demands for a large capacity and a high speed for interchange and transmission of information data. In order to meet such demands, techniques relating to radio communication are being rapidly developed.
The radio communication is now spreading in offices and homes because there is no need to provide cables and the like. In particular, UWB communication expected for communication with small power consumption is regarded as a promising communication method.
In the UWB communication, ultra wide band signals utilizing a frequency band of 1.5 GHz or more or a frequency band exceeding 25% of the center frequency are used. As one communication system of the UWB communication, an MB-OFDM (multiband orthogonal frequency-division multiplexing) system in which a carrier frequency is hopped every given period has been proposed.
FIG. 6 is a diagram for showing an exemplified conventional radio communication system. The architecture shown in FIG. 6 employs the MB-OFDM system (see Akio Tanaka, Hiroshi Kodama, Akifumi Kasamatsu, “Low Noise Amplifier with Center Frequency Hoping for an MB-OFDM UWB Receiver”, Ultra Wideband systems, 2004, Joint with Conference on Ultrawideband Systems and Technologies, Joint UWBST & IWUWBS. 2004 International Workshop on, IEEE, Mar. 18-21, 2004, pp. 420-423). In FIG. 6, a reference numeral 91 denotes an antenna, a reference numeral 92 denotes a band pass filter (BPF) of a narrow band, a reference numeral 93 denotes a low noise amplifier (LNA) capable of hopping a gain frequency band, a reference numeral 94 denotes a balun for converting a single input into a differential signal, reference numerals 95a and 95b are mixers, a reference numeral 96 denotes a demodulator unit and a reference numeral 97 denotes a local oscillator (LO) capable of hopping a frequency.
In a receiving operation, the antenna 91 receives a signal from a transmitter antenna, and the signal transferred through the band pass filter 92 is amplified by the low noise amplifier 93. Thereafter, the signal is subjected to single-differential conversion by the balun 94, and the resultant signal is shifted in the frequency by the mixers 95a and 95b and the local oscillator 97, so as to be demodulated by the demodulator unit 96.
At this point, in the MB-OFDM system for the UWB, for hopping the carrier frequency, the gain frequency band of the low noise amplifier 93 is also hopped for amplifying the signal in a selected band. In this manner, a high gain is realized as compared with the case where a signal of a wide band is amplified, and thus, a good communication system is obtained.
In the conventional method, however, there is a problem that the communication performance is degraded when the frequency band of a signal amplified by the low noise amplifier and the hopping frequency of the local oscillator are shifted from each other. Furthermore, it is necessary to transmit a pilot signal from a transmitter for correcting the shift, and hence, it disadvantageously takes time to attain synchronization for starting communication.